Method and apparatus for detecting flaws in magnetizable bodies



April 27, 1943. w c, BARNES r AL 2,317,719

METHOD AND APPARATUS FOR DETECTING FLAWS IN MAGNETIZABLE BODIES OriginalFiled June 8, 1935 5 Shee ts-Sheet l Jill/anions Wal er GEM/Zea fierzryWfew'l April w. c. BARNES arm. 2,317,719

METHOD AND APPARATUS FOR DETECTING FLAWS IN MAGNETIZABLE BODIES OriginalFiled June 8, 1955 5 Sheets-Sheet 2 90 I zzirizz IIM lllllH Illl llllli\ mmmummmmmm i mum!mamumhmmhui Zia/anions Walter C. 1301 7225 172 n yY/JZ w) April 27, 1943. w. c. BARNES ETAL 7,7

METHOD AND APPARATUS FOR DETECTING FLAWS IN MAGNETIZABLE BODIES OriginalFiled June 8, 1935 5 Sheets-Sheet 25 April 27, 1943. w. c. BARNES arm.2,317,719

METHOD AND APPARATUS FOR DETECTING FLAWS IN MAGNETIZABLE BODIES OriginalFiled June 8, 1935 5 Sheets-Sheet 4 April 27, 1943. w. c. BARNES ETAL2,317,719

METHOD AND APPARATUS FOR DETECTING FLAWS IN MAGNETIZABLE BODIES OriginalFiled June 8, 1955 5 Shee ts-Sheet 5 Patented Apr. 27, 1943 METHOD ANDAPPARATUS roa DETECTING- FLAWS IN MAGNETIZABLE BODIES Walter C. Barnes,Lake Bluff, and Henry W. Keevil, Highland Park, 111.

Original application I 25,586. Divided and 1940, Serial No. 330,210

9 Claims.

This application is a division of our copending application, SerialNumber 25,586, filed June 8, 1935.

In Sperry Reissue Patent No. 18,555, the following statement appears(page 1, lines 12-35) "A method of detecting fissures has beenheretofore proposed wherein the magnetic properties of the rail isutilized. According to this prior method the rail is placed in a strongmagnetic field which ismoved relatively to the rail. and means areemployed to detect variations in the field produced by the varyingpermeability of the rail from the point to point. The theory was that ahidden fissure would cause a variation in the number of lines of forcepassing through the object which could be detected by the re- I sultantvariation in the field surrounding the object, but the method failedbecause variations in such field were produced not only by fissures butoften more pronouncedly by other variations in the physicalcharacteristics of the rail, such as hard spots, soft spots, hammerblows, gag marks, etc. Such a method, therefore, failed of its purpose,for it was impossible to distinguish between fissure indications andthose due to other causes which do not seriously weaken the rail.

Others have regarded magnetic testing with the same distrust. Forexample, Sanford and Kouwenhoven in writing on the subject of "Locationof Flaws in Rifle Barrel Steel by Magnetic Analysis (Proceedings of theAmerican Society for Testing Materials (1919) vol. 19, part 11, page 81)state, at page 92:

"The greatest dimculty in this line of investigation lies in theinterpretation of the results. This is due to the fact that there aremany causes which may produce magnetic inhomogeneity and it is difiicultto differentiate between them.

Several years later, Mr. Sanford, in an article entitled NondestructiveTesting of Wire Hoisting Rope by Magnetic Analysis (published inTechnologic Papers of the Bureau of Standards, No. 315, April 16, 1926)makes the following statement (pages 517-518) Slight variations instress conditions along the length of a specimen, therefore, often giverise to irregularities in the records of magnetic exploration testswhich can not be distinguished from those resulting from flaws.

There are as yet so many sources of uncertainty, however, that thereseems to be no immediate prospect of the development of a magneticmethod for the inspection of wire rope une 8, 1935, Serial No. thisapplication April 17,

of a suiiicient degree of reliability to warrant its use on a commercialbasis."

We have discovered that the uncertainty in magnetic testing has beendue, not to the lack of correlation between magnetic properties andmechanical properties, butto the fact that the detection of flaws hasinvariably been attempted in the presence of the energizing field. Thatis the reason that the results obtained were neither reliable norcapable of exact interpretation, for certain variables were unavoidablypresent which greatly affected the final result. For example, variationsin the energizing iield, slight changes in the air gap betweentheelectromagnet and body under test, slight diflerences in hardness andreluctance of diflerent portions of the test piece, were responsible formuch of the difilcuity encountered in magnetic testing as it hasheretofore been attempted.

According to our invention, relatively large variations in the strengthof the energizing flux, the extent of the air gap, the hardness of thematerial, and the like, have no effect on the record obtained, thesesources of uncertainty being entirely eliminated by using residualmagnetism for indicating inhomogeneities in the test material.

Residual magnetism has been used by some investigators for locatingsurface cracks or imperiections in metallic bodies, the cracks ordefects being indicated by the characteristic pattern taken by finelydivided magnetic particles, such as iron filings or the like, sprinkledover the surface of the body. This method has never been successful inlocating deep internal defects and the method, of course, is entirelyunsuited for the progressive testing of rails in track.

In our method, we first set up a relatively strong magnetic field inaportion of the body under test to cause it to be uniformly magnetizedin a given direction and to overcome any magnetic condition that mayhave previously existed in that portion of the body.

We then remove the energizing field and search the space around the bodyfor traces of residual magnetism. because we have found that cracks,fissures and such like cause a peculiar magnetic condition to be set upthat can be detected externally by suitable means.

The more important objects of our invention. therefore, are to provide amethod and apparatus for detecting flaws in magnetizable bodiesutilizing the residual magnetic condition in the vicinity of flaws tolocate the flaws; to disassociate the detection apparatus from theenergizing field so that the detection apparatus is entirely independentof changes in energizing force, hardness of material, and other factorswhich heretofore have caused uncertainty in the interpretation ofrecords obtained by conventional methods of magnetic analysis; to adaptour method of testing to locating flaws in track; to provide a test carequipped with apparatus for using our method, and which is capable ofcontinuously testing the track over which the car operates while the caris moving at a speed of several miles an hour, or more; and toaccomplish these results with apparatus that is substantially lessexpensive, but more reliable and accurate than apparatus on cars now inuse.

The present application relates particularly to a modified form of theinvention claimed broadly in Ser. No. 212,121 which is to issue as apatent on the same date as this application.

Further and other objects and advantages will become apparent as thedisclosure proceeds and the description is read in conjunction with theaccompanying drawings, in which Fig. 1 is a diagrammatic, sideelevational view of a test car, with one form of apparatus for carryingout our invention;

Fig. 2 is a longitudinal, sectional view through the support casting forthe detector shoe, the section being taken on the line 2-2 of Fig. 3;

Fig. 3 is a section taken on the line 33 of Fig. 2;

Fig. 4 is a plan view of a detector unit employing two longitudinallyarranged detector coils connected in opposition, the top cover beingbroken away to expose the coils;

Fig. 5 is a longitudinal, sectional view taken on the line 5-5 of Fig.4;

Fig. 6 is a transverse, sectional view taken on the line 66 of Fig 4;

Fig. 7 is a perspective View of the core for one of the coils shown inFigs. 4 and 5;

Fig. 8 is a diagrammatic, perspective view i1- lustrating one embodimentof the invention;

Fig. 9 is a view that will be used in explaning the theory which isthought to account for the advantageous. results obtainable with ourmethod;

Fig. 10 illustrates the magnetic condition at a broken rail;

Fig. 11 is a diagrammatic view showing the manner in which an electricalcurrent may be passed longitudinally through the body under test forsetting up the magnetic field, this arrangement being particularlyadvantageous for locating vertical split heads;

Fig. 12 is a sectional view taken on the line |2l2 of Fig. 11;

Fig. 13 shows the type of coil that is most suitable for use indetecting fissures of the type shown in Figs. 11 and 12; v

Fig. 14. shows another method that is particularly suitable for locatingsplit heads;

Fig. 15 is a sectional view taken on the line l5-I5 of Fig. 14;

Fig. 16 illustrates an arrangement in which current is passedtransversely through the rail head, and a longitudinal exploring coil isutilized for locating the fissure;

Fig. 17 is a sectional view taken on the line |1|1 of Fig. 16;

Fig. 18 is a sectional view through the fissure shown in Fig. 16indicating the distribution of flux;

Fig. 19- is an end view of the detector coil shown in Fig. 16;

' been chosen for the Fig, 20 shows an arrangement that is particularlysuitable for locating compound fissures;

Fig. 21 is a sectional view taken on the line ZI-Zl of Fig. 20;

Fig. 22 illustrates the probably fiux distribution in the region of thecompound fissure;

Fig. 23 is a sectional view taken on the line 23-23 of Fig. 20;

Fig. 24 is a view showing the manner in which transverse coils when usedin pairs are preferably connected together;

Fig. 25 shows the manner in which longitudinal coils when used in pairsare preferably connected together;

Fig. 26 is a sectional 25-25 of Fig. 1;

Fig. 27 is a sectional 2'i2'i of Fig. 1; and

Fig. 28 indicates the manner in which two cars may be used for testingtrack, one carrying the means for setting up the magnetic field in therails, and the other carrying the detection equipment.

The embodiments of the invention shown in the drawings and hereinafterdescribed have purpose of making the disclosure required by sec. 4-888of the Revised Statutes, but it will be understood that the scope of theinvention is not limited to the specific embodiments shown anddescribed.

In the preferred form of the invention, a large electromagnet passesuni-directional magnetic flux through a portion of the body under testand the body is then explored for traces of residual magnetism by aninduction coil suitably connected to amplifying and recording apparatus. This form of the invention is illustrated somewhat diagrammaticallyin Figs. 1-6 inclusive and will first be described.

The flaw detecting mechanism is mounted in a detector car, indicated at50, comprising an underframe 5i mounted on wheeled axles 52 and 53 andsupporting a house body 54, the forward end of which is adapted to housethe necessary equipment for operating the car, the rear end beingreserved for apparatus that is used in conjunction with the flawdetector mechanism.

The car is preferably powered by a governor controlled internalcombustion engine 55 of approximately 60 H. P. capacity which transmitspower through a clutch and transmission 56, drive shaft 57, gear box 58and chain belts 59 to the axles 52 and 53 of the car. The lever 60indicates an operating means for the clutch and transmission 56, thelatter including a reverse gear for running the car backward.

The speed of the engine 55 and the gear ratio of first gear in thetransmission 56 are such that the car may be driven forward at someconstant predetermined speed, for example five miles per hour for makingtest runs.

Mounted between the axles 52 and 53 is a. carriage 6! adapted to supporta relatively large electromagnet 62 consisting of an iron core 53 andcoils 64 that are energized by a generator 65 (preferably volt, 2 k. w.direct current), driven by suitable means, as for example, from a powertake-01f 66 associated with the transmission 56. A clutch 61 operated bya lever 68 enables the generator to be disconnected from its drivewhenever desired.

The source of power for the car and for driving the generator 65 is somuch a matter of choice that further description is deemed unnecessary.

view taken on the line view taken on the line The electromagnet carriage8| is equipped with flanged wheels 18 adapted to ride upon the track 1|with the wheel flanges in engagement with the gauge side of the rail.When the car is not being used for detection purposes, it is desirableto lift the electromagnet carriage from the rail, and this isaccomplished by mounting the carriage in channel guides 12 which dependfrom the car underframe The carriage is lifted by a piston 18 operatedby compressed air and connected by cables 18 to the electromagnetcarriage. There is a lost motion connection (Fig. 26) between thecarriage 5i and the arms 18 which travel in the guides 12 so that whenthe carriage is lifted by the cable 18, it moves toward the guides 12.The purpose of this arrangement is to cause the carriage to be loweredin such a manner that the flanges of the wheels 18 will always engagethe gauge side of the rail instead of riding on the rail. Tensionsprings 11 attached atone end to the carriage and at the other end tolugs 18 hold the wheel flanges against the gauge side of the rail whenthe carriage has been lowered, the angularity of each spring, and itstension, being such that it only comes into play after the carriage hassubstantially reached its operative position.

The detector unit generally designated 88 is mounted beneath the rearend of the car and like the electromagnet carriage, is adapted to belifted from the rail when the flaw detection apparatus is not beingoperated. The detection unit is pivotally supported from arms 8iextending downwardly from the car underframe, the connection between thedetector carriage 82 and the arms being through links 83 equipped withcam slots 88, adapted to loosely receive carriage studs 85, as bestshown in Figs. 1 and 2'1.

The carriage 82 is preferably made of brass and is equipped with areplaceable manganese steel runner 88 (Figs. 2 and 3). The body 81 ofthe carriage has a flange 88 which overhangs the gauge side of the railand fixes the position of the detector shoe with respect to the railhead.

The central portion of the carriage 82 is recessed to receive thedetector shoe 88 (Fig. 2) which may be supported from the carriage bycarriage bolts 88 which loosely pass through slots 8| in overhangingwalls 82 of the carriage. Nuts 83 screw on to the projecting ends of thebolts 88 and cooperate with springs 84 interposed between the walls 82and the detector shoe 88 for supporting the latter in proper positionover the rail head. This particular mounting has the advantage that itenables the detector shoe to be adjusted both vertically and laterallywith respect to the rail head.

When the detecting unit consists of inductive means, it is preferable toemploy two longitudinally arranged coils 85 and 88 placed end to end andconnected in opposition (that is so that any longitudinal flux, orcomponent of flux that simultaneously acts on both coils will produce E.M. Fs that oppose each other and consequently balance out). Each coil ismounted on an H-shaped core 81 (Fig. '1), the four legs of which areprovided with downwardly extending feet 88. The cores are preferablylaminated as best shown in Fig. '1 in order to better conductlongitudinal components of flux through the coils. The coils and theircores are mounted,

in recesses 88 provided in the detector shoe, the latter beingpreferably of Bakelite or some other insulating material and comprisinga base I88 and cover IN, the latter being removably secured to the baseby countersunk machine screws I82. The partition I88 between therrcesses 88 receives the binding posts I88, as best :;.\own in Figs. 4and 5 to which the terminals of the coils and 88 are connected. Suitableleads connect the binding posts I88 with amplifying apparatusconveniently mounted in a compartment I85 at the rear of the car.

The carriage 82 is lifted to inoperative posi-- tion by a cable I88attached at one end to the carriage and at the other end to the piston18. When the carriage is raised, the cam slot 88 causes the carriage tomove to the right (Fig. 2'!) so that when it is thereafter lowered, theflange 88 will be sure to fall inside of the gauge edge of the rail.Relatively light compression springs I81 extending between the arms 8|and the carriage 82 have their angularity and strength such that whenthe carriage has been substantially lowered to operative position, thesprings will force the flange 88 of the carriage against the gauge sideof the rail.

Referring now to Figs. 8-10 inclusive, an attempt will be made toexplain the theory which is believed to underlie the present inventionalthough all theoretical discussions in this speciflcation are to beconstrued not as defining a mode of operation, but merely as a possibleexplanation of certain physical, electrical or magnetic phenomena knownto exist.

In the latter part of 1918 and early part of 1919, considerable work wasdone on magnetic analysis by Charles W. Burrows and Frank P. Fahy, atthe Bureau of Standards, this work being reported in the Proceedings ofAmerican Society for Testing Materials, 1919, vol. 19, part II,.pages5-49. One of the methods discussed by Messrs. Burrows and Fahy wascalled the magnetic leakage method, and it consisted essentially ofmeasuring directly the variation in magnetic flux along the length ofthe rail. The authors, however, admit that it is difficult to produce auniform magnetizing force along any considerable length of a rail, andthe results of their experiments clearly show that hard and soft spotsin the rail have a very substantial effect upon the record that is made.

All of the Burrows and Fahy work was based on the detection of magneticleakage in the presence of the energizing field, and it was because ofthis basis that the method was not commercially successful, forobviously variations in the magnetizing force, the air gap and thehomogeneity of the metal produced substantial indications that were notdistinguishable from those produced by flaws.

In our method, the detection of flaws is done after the energizing fieldhas been collapsed, or has been moved to another portion of the bodyunder test. The indication that is produced by this method is caused notby leakage of the energizing flux, but by a condition of residualmagnetism that is set up at discontinuities, or flaws, in the rail.

Let us now refer to Fig. 9 to see what happens when a longitudinallyarranged electromagnet is passed over a transverse fissure and isfollowed in its course by a longitudinally arranged induction coil.Assuming that the magnet 8 shown in Fig. 9 is energized substantially tosaturation by uni-directional flux, we may assume that the flux willtravel through the rail somewhat in the manner indicated by the fluxlines III. If, while the electromagnet is being moved along the rail, itpasses a transverse fissure, such as one face of the fissure to become anorth pole and the other face a south pole, and the striking phenomenonis that this condition persists even after the electromagnet I I hasmoved beyond the fissure. This residual magnetic condition is manifestedon the exterior of the rail by'fiux lines extending outside of the railhead somewhat as indicated by the fiux lines l I3. These fiux lines lieessentially in planes which include the center line of the rail andhence when a longitudinally arranged induction coil H4 is passed throughthe field, a current will be induced in the coil WhlCh may be suitablyamplified at H5 and recorded at II6 to indicate the presence of thefissure.

Theoretically, it is unnecessary to use two detector coils connected inopposition because the detection unit is not operating in the presenceof the energizing field, but it is preferable in order to balance outstray fields which might otherwise produce an indication. In Fig. 8,there is shown diagrammatically an application of the method to adetector car. The wheeled axles 52 and 53 of the detector car are shownwith direct current electromagnets II 0 between the axles andlongitudinal pickup coils H4 in rear of the rear axle. A direct currentgenerator is indicated at II1 for energizing the electromagnets IIO, theelectromagnets being connected in parallel and controlled by a rheostatII8. Switches H9 and I20 are provided for separately controlling theparallel circuits.

The right rail pickup I I4 is connected by leads I2I and I22 to anamplifier I23, preferably condenser coupled and having a sensitivitycontrol I24 in the grid circuit of the first tube I25. The left railpickup I I4 is connected by conductors I26 and I21 to a similaramplifier I28 which also has a sensitivity control I29 in the gridcircuit of the first tube I30. The amplifier circuit will be understoodby those skilled in the art and further description is deemedunnecessary.

The output of the amplifier I23 is in series with a pair of coils I3Iand I32 which are adapted to raise front contact armatures I33 and I34,respectively, when sufiicient current passes through the coils. Thearmatures I33 and I34 are in a local circuit which includes a batteryI35 and pen relays I36 and I31. A record strip I38 is moved continuouslyby suitable means beneath the pen relays, either in proportion to car'speed, or at some predetermined speed which may be varied to suitconditions of testing. The pens I39 and I40 associated with the penrelays I36 and I31 cause record lines I4I and I42 to be transcribed onthe record sheet. Preferably, one of the relays in the amplifier outputcircuit has a lower pickup value than the other so that a relativelyweak current through the circuit will operate only one of the relays(for example coil I3I with its armature I33), whereas a somewhatstronger current will operate both relays. The jogs I43 and I44 in therecord lines, therefore, having been produced (in this illustration)simultaneously by the same current impulse, indicate that the detectingunit has traversed a relatively strong residual magnetic field.

In a similar manner, the output from the amplifier I28 associated withthe left rail pickup will energize coils I45 and I46, and operate penrelay I41, or pen relays I41 and I48, according to the strength of theamplifier output current, thereby indicating by the record lines I49 andI 50 the induction of a current in the left rail pickup II4.

In Fig. 10, there is illustrated diagrammatically the residual magneticfield that exists at a broken rail, and it is obvious that the breakI5I, after it has been traversed by a magnetic fiux will leave aresidual field somewhat as shown in this figure. Consequently when adetector coil, such as I52 passes through the field, the [broken railwill readily be detected. As a matter of fact, a broken rail willproduce an exceptionally strong indication whereas in theelectro-inductive method of testing shown in Sperry Reissue 18,555,considerable difiiculty was experienced in locating broken rails, as isevidenced by Drake 1,907,756, which dealt with that particular problem.

The combination of a longitudinally arranged electromagnet and alongitudinal coil is particularly suitable for detecting transversefissures and it will later appear that other combina tions are moreefilcient in locating other types of fissures.

There are other ways of setting up a magnetic fiux in a rail andlocating residual magnetic fields by induction means, and some of theseare shown in Figs. 11-23 inclusive.

In Fig. 11, a combination is shown that is particularly suitable forlocating a vertical split head, as indicated at I60. Here, a relativelystrong, direct current is passed through the rail, the source of currentbeing indicated at I6I, and the brushes for applying the current to therail being indicated at I62 and I63. The current in passing through theportion of the rail between the brushes I 62 and I63 sets up arelatively strong magnetic fiux that is transverse of the rail head,this fiux being indicated diagrammatically at I64. This fiux isinseparable from the current which produces it and only exists betweenthe brushes I 62 and I63. Behind the brush I 63, there is no magneticfield except where the flux has traversed a fissure such for example asshown at I60 where the faces of the fissure have become magnetized and aresidual field exists as indicated at I 65. The lines of flux in thisfield are transverse of the rail head and a transverse induction coil,such as indicated at I66 is best suited for detecting the presence ofthe residual field.

Fig. 12 shows the vertical split head in cross section and the magneticfield associated with it, and Fig. 13 shows the transverse coil I66 inits relation to the magnetic fiux.

Instead of passing an electrical current through the rail, as shown inFig. 11 in order to produce a transverse flux, the flux may be produceddirectly by a direct current electromagnet I61, (Figs. 14 and 15) thecoil I68 of which is energized by a suitable source I69. The residualfield at the split head I60 is the same as shown in Figs. 11 and 12 andthe same pickup I66 may be used. The core I 10 of the electromagnet ispreferably U-shaped with the poles turned inwardly in order to apply theflux transversely of the rail without interference from switch frogs,joint bars, and the like.

In Fig. 16, there is shown an alternative for the arrangement shown inFigs. 1, 8 and 9. In this case, the longitudinal flux is set up by anelectrical current that is passed transversely through the rail bybrushes HI and I12 (Figs. 16 and 17) energized by a suitable source ofdirect current I13. The current produces a flux I14 longitudinally ofthe rail, and this flux is particularly suitable for producing aresidual magnetic condition at transverse fissures, such as indicated atill. Since the lines of force in the residual magnetic fiux I at thefissure I lie in planes which include the longitudinal axis of the rail,it is preferable to employ a longitudinal exploring coil i'I'I in orderto obtain the greatest possible indication from the flux at the fissure.

In Fig. 20, a compound fissure is indicated at I18, that is a fissurewhich has a horizontal and transverse component. The magnetization ofthis type of fissure may be effected by a longitudinally placed directcurrent electromagnet, such as indicated at I19, which sets up a fieldilii through the rail. It will be noticed that this field I80necessarily has a vertical, as well as a horizontal component, and theformer is particularly effective in magnetizing the horizontal portionof the fissure I18 and the longitudinal component is particularlyeffective in magnetizing the vertical portion.

Since the residual field ill set up by the horizontal portion of thefissure has relatively strong longitudinal, transverse and verticalcomponents, and since the field I82 set up in the region of thetransverse portion of the fissure has relatively strong longitudinal andvertical components, it is desirable to employ an induction coil I"which is placed with its axis at an angle to all three principal axes ofthe rail, as best shown in Figs. 20 and 23. In this position, the coilwill always out some lines of force and an indication of the defect inthe rail is, therefore, assured. In this arrangement, a second coilconnected in series opposition to the first coil and making differentangles with respect to the three principal axes of the rail would bedesirable.

Figs. 24 and 25 merely show that when two transverse coils are used,they are preferably placed side bv side and connected in the mannershown, that is with the coils connected in series but wound in oppositedirections. When two longitudinal coils are used, they are placed intandem and connected in series but oppositely wound.

Although Fig. 1 shows the energizing electromagnet and the detector unitmounted on the same car, it is obvious that the magnetization of therail may be effected by one car as indicated at I84, and the detectionof flaws by another car, as indicated at I85. (Fig. 28) v The energizingflux produced by the electromagnet, or other source of fiux, should besufficient, preferably, to wipe out all residual magnetism previously inthe rail, and set up a substantially uni-directional fiux through therail. To do this, the electromagnet, if that is the source of fiux,should have a cross section that is sufilciently large in relation tothe flux setup by the energizing coils 54 (Fig. 1) that thecore 63 doesnot became saturated for if it does, the amount of stray field isconsiderably increased, and the detection unit may be affected.

We claim:

1. In a car ior locating flaws in track, means for setting up auni-directional, transverse, magnetic flux through a limited portion oftrack, and a detector unit on the car located in rear of said means andoutside of its field of operation whereby when the car is moved forwardthe detector unit passes progressively over a portion of the track thathad been previously magnetized by said means, said detector meansincluding an induction coil positioned with its longitudinal axistransverse of the rail, whereby it is responsive to the residualmagnetism set up by the transverse magnetizing means at internal andcracked-out vertical split heads.

2. The method of detecting an internal fiaw in a ferro-magnetic bodyhaving one of the flaw faces so positioned that a line normal theretolies in a vertical plane which is at an angle to the longitudinal axisof the body, which consists in subjecting a portion of the body toexternally applied forces causing transverse lines of fiux in the bodyand thereby establishing a residual magnetic field in the vicinity ofsaid fiaw, removing all of said forces from said portion of the body,and subsequently not simultaneously locating said field by amagnetically responsive device moved along the body and constructed andarranged to detect the presence of magnetic lines of force having acomponent transverse to the body.

3. The method of detecting fiaws in a ferromagnetic body which consistsin subjecting the body to the energizing force which, when removed,leaves a residual magnetic field in the vicinity of said fiaws, thenlocating said field by moving a flux responsive device along said bodyto cause the device to produce an indication when it traverses saidfield, said device including a detector coil having its longitudinalaxis at a zigbstantial angle to the longitudinal axis of the 4. Themethod of detecting fiaws in a ferromagnetic body which consists insubjecting the body to the magnetic field set up by an electricalcurrent progressively introduced between spaced points in the bodywhereby a residual magnetic field is established in the vicinity of thefiaw and then locating said field by moving a flux responsive devicealong the body in a path that will cause the device to produce anindication when it traverses said field, said device including aninduction coil having its axis substantially normal to the direction ofcurrent fiow.

5. The method of detecting fiaws in a ferromagnetic rail which consistsin subjecting. the rail to the magnetic field set up by an electricalcurrent progressively introduced between longitudinally spaced points inthe rail whereby a. residual magnetic field is established in thevicinity of the flaw and then locating said field by moving a fiuxresponsive device along the rail in a path that will cause the device toproduce an indication when it traverses said field, said deviceincluding an induction coil having its axis substantially normal to thedirection of current fiow.

6. The method of detecting fiaws in a ferromagnetic rail which consistsin subjecting the rail to the magnetic field set up by an electricalcurrent progressively introduced between longitudinally spaced points inthe rail whereby a residual magnetic field is established in thevicinity of the fiaw and then locating said field by moving a fiuxresponsive device along the rail in a path that will cause the device toproduce an indication when it traverses said field, said deviceincluding an induction coil having its axis at a substantial angle tothe longitudinal axis of the rail.

7. The method of detecting flaws in a ferromagnetic rail which consistsin subjecting the rail to the magnetic field set up by an electricalcurrent progressively introduced between longitudinally spaced points inthe rail whereby a residual magnetic field is established in thevicinity of the fiaw and then locating said field by moving a fluxresponsive device responsive to flux transverse of the rail along therail in a path that will cause the device to produce an indication whenit traverses said field.

8. The method of detecting flaws in a ferromagnetic bodv which consistsin subjecting the body to the magnetic field set up by an electricalcurrent progressively introduced between spaced points on the bodywhereby a. residual magnetic field is established in the vicinity of theflaw and then locating said field by relatively moving a flux responsivedevice responsive to fiux transverse to the direction of current flowalong the 15 the body.

WALTER C. BARNES. HENRY W. in:

to produce an indication

